La Crosse Technology 2800 Weather Radio User Manual

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RELATIVE(AIR)PRESSURE

WHATISRELATIVEPRESSURE?

Relative(alsoreferredtoasatmospheric)pressureisdefinedastheforceperunitareaexertedagainstasurface

bytheweightofairabovethatsurfaceatanygivenpointintheEarth’satmosphere.

Basically,eventheatmospherehasweight.Formostpurposes,wecangetairpressurebymeasuringthe

hydrostaticpressurecausedbythe

weightofairaboveacertainpoint.

Lowpressureareashavelessatmosphericmassabovetheirlocation,whereashighpressureareashavemore

atmosphericmassabovetheirlocation.Aselevationincreasesthereislessoverlyingatmosphericmass,sothat

pressuredecreaseswithincreasingelevation.

AIRPRESSUREANDWEATHERFORECASTING

Airpressureiscommonlyusedforweatherprediction,asaregionofhighairpressureindicatesfairweather,while

lowpressuretellsusthatrainorstormsaremorelikely.Whenusedincombinationwithwindobservations,

reasonablyaccurateshorttermforecastscanbemade.

Eachgeographicareahasitsown

‘normal’rangeofairpressurevalues.Youcanobtainthisinformationfromyour

localweatherreportingstation(e.g.theairport).Withthatinformation,youcancalibratetheweatherdisplayto

thereportingstation(seeOperationsManualforaccessingtheSETmenu).Whencalibrated,therelativeincrease

anddecreaseinair

pressurevalueswillcorrelatewithyourlocalmediareports.Ifyoudoso,rememberthatany

hardwareresetwillrequiretheweatherdisplaytobere‐calibrated.

Simultaneousrelative/airpressurereadingsfromacrossanetworkofweatherstationsallowmapsofairpressure

areastobeproduced,whichwerethefirst

formofthemodernweathermapwhencreatedinthe19thcentury.

Isobars(linesofequalpressure),whendrawnonsuchamap,givesacontourmapshowingareasofhighandlow

pressure.

Localizedareasofhighatmosphericpressureactasabarriertoapproachingweathersystems,divertingtheir

course.Lowatmosphericpressure,ontheotherhand,representsthepathofleastresistanceforaweather

system,makingitmorelikelythatlowpressurewillbeassociatedwithincreasedstormactivities.

Typicallywithpersonalweatherforecasting,wefocusonthechangeinairpressureovertime.Thelargerthe

change

inpressure,especiallyifmorethan3.5hPa(0.10inHg),thelargerthechangeinweathercanbeexpected.

Ifthepressuredropisrapid,itindicatesthatalowpressuresystemisapproaching,andthereisagreaterchanceof

rain.Rapidpressurerisesareassociatedwithimprovingweather

conditions,suchasclearingskies.

RELATIVEPRESSUREALARM

Youcanconfigureanalarmtosoundwhentherelativepressuregoesaboveorbelowacertainpoint.Formore

detailsonhowtosetanalarm,seethesectiononSettingAlarms

.



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La Crosse Technology 2800 Weather Radio User Manual